blender bge have chosshairs in center of viewport
In the top navigation bar of Blender you can see a dropdown including the word "Scene". That's the scene you're currently in. Press the + sign to create a new one and call it "HUD" for example. In this scene you put a camera at the center looking towards the grid floor. Put the camera in orthographic view. Press 0 on the numpad to look through the camera and place a crosshair at the center as you like.

Now switch back to your default scene and add a logic brick with 


Sensor: always (activate "true level triggering" to run each tick)
Controller: And
Actuator: Scene


On the actuator/scene brick change the mode to "Add overlay Scene" and for "scene" select the scene that you have created for your hud.

Press P to start the game and you should see your crosshair.